Path: csiph.com!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Mars lander crash
Date: Tue, 13 Dec 2016 08:19:37 -0800
Organization: None to speak of
Lines: 25
Message-ID: <lnr35bpzs6.fsf@kst-u.example.com>
References: <o2e7s1$ea6$1@dont-email.me> <o2e8sg$hp7$1@dont-email.me> <o2ecu8$v73$1@dont-email.me> <o2eel2$614$1@dont-email.me> <o2eg20$b90$1@dont-email.me> <o2emds$5ek$1@dont-email.me> <o2epuf$juu$1@dont-email.me> <o2es0l$sea$1@dont-email.me> <o2jgbu$ih7$1@dont-email.me> <ln7f76utpn.fsf@kst-u.example.com> <o2kkmj$fbj$1@dont-email.me> <9e80bbb3-a034-4f3a-8c7e-a58387e616a4@googlegroups.com> <o2losm$ct1$1@dont-email.me> <eb78evF156qU1@mid.individual.net> <0fdbff52-14ad-4566-8034-70432db815f3@googlegroups.com> <eb79ggF156qU2@mid.individual.net> <65385380-f423-4276-9d27-0dcbe582af39@googlegroups.com> <eb8bj5F18c4U1@mid.individual.net> <13b758b6-d938-4ffd-9659-c76a0007ba62@googlegroups.com> <eb90bsF18c4U2@mid.individual.net> <bd14a6ca-b4e9-4685-abcd-22765b96acd8@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Injection-Info: mx02.eternal-september.org; posting-host="dfa63d714b7efbf23555ba45297b3f3c"; logging-data="30391"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/8xs9h4UzRRGnS3TsNR3yy"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
Cancel-Lock: sha1:X/6Rwpop5lyBuR8eP4Xy8gPycD4= sha1:4KB+XAGeVlurDjRmmRQcIOQ2wXs=
Xref: csiph.com comp.lang.c:99215

Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
[...]
> One way of solving the curve intersection problem is to build
> a root finder which will solve equations with nine roots.
> If the root finder works for all representable values of
> coefficients, then great. But what if it doesn't? Is to going
> to be good enough, (2 cubics can intersect nine times, but
> almost always they intersect only once or twice on the interval
> you are interested in)?  

Two cubics can intersect nine times?  I don't think so.

A cubic is an equation of the form ax^3 + bx^2 + cx + d = 0.
It can have up to three roots (values of x for which it's true).
The intersections of two cubics are the solutions of the difference
between the two cubics -- which is itself a cubic, which have no
more than three solutions.

Is there a confusion about terminology?

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"